[feature] support exporting alerts to Excel

[orangeCatDeveloper]

Fixes #4212

The alarm center could display alerts but offered no way to export them, so users had no way to pull alert data out for offline statistics.

Add an "Export Alerts" button to the alarm center that downloads the matching alerts as an .xlsx file. The endpoint reuses the alert list's filter logic (status + fuzzy search), extracted into a shared buildSingleAlertSpecification, so the export always matches what the user currently sees. The matching single alerts are flattened for a tabular sheet — labels/annotations maps joined to k=v; ... strings, epoch-millis timestamps formatted to dates — and rendered via easypoi. Default sort is activeAt desc so the ordering column is visible in the sheet.

The export is an unpaged full-result query, same as the existing monitor export — acceptable for typical alert volumes.

Verification

End-to-end, three scenarios (3 seeded alerts: 2 firing + 1 resolved):

default export  -> 3 rows, sorted by activeAt desc:
    firing   e2e-fp-mem  Active At 2024-12-12 20:11:40
    firing   e2e-fp-cpu  Active At 2024-12-12 20:11:17
    resolved e2e-fp-disk Active At 2024-12-12 18:40:00
status=resolved -> 1 row  (e2e-fp-disk)
search=server-02 -> 1 row (e2e-fp-mem)
output file type: Microsoft OOXML (valid .xlsx / XSSF)

UI — the new Export Alerts button in the alarm center: